A gas sensor (aka, gas detector) is a device configured to detect the presence or absence of one or more target gases in a gaseous environment (e.g., a volume of air). For example, gas sensors are used to detect dangerous (e.g., flammable or toxic) gases in amounts that exceed minimum safety levels, or to detect oxygen depletion (i.e., when ambient oxygen levels fall below a predetermined concentration level). Gas sensors typically interface with a safety control system that performs a safety function in response to a positive detection signal generated by a gas sensor (e.g., to automatically shut down a process, or to sound a safety alarm, when the amount of target gas exceeds or falls below a predetermined concentration level).
Gas sensors capable of quantitatively measuring the concentration level of a target gas generally include remote-type gas sensors and contact-type gas sensors. Remote-type IR sensors, which include infrared (IR) point sensors and IR imaging sensors, are capable of detecting a target gas in a specified environment (i.e., the gas-filled volume containing the gas) without being in physical contact with the target environment, and are typically used to detect or measure gas leaks in large area environments such as oil refineries. However, remote-type gas sensors are typically expensive to produce and operate, and are thus impractical for detecting target gasses in smaller enclosed areas. In contrast to remote-type gas sensors, contact-type gas sensors are placed in direct contact with a monitored environment, and utilize gas sensing elements that react in a measurable way when a target gas is present in the environment. Contact-type gas sensors are typically smaller and less expensive than remote-type gas sensors, and are utilized mainly in enclosed areas such as buildings or processing chambers.
Semiconductor gas sensors are contact-type sensors including gas sensing elements whose electrical resistance changes in response to a reaction caused by the presence of a target gas, whereby detection or measurement of the target gas is achievable by way of monitoring changes in a current passed through the gas sensing element. The resistance change of gas sensing elements in semiconductor gas sensors is typically caused by one of three different reaction types: (i) a chemical reaction caused by changes in the composition or chemical structure of the gas sensing element in response to adsorption of the target gas into the gas sensing element; (ii) a temperature change of the gas sensing element as a result of endothermic or exothermic (e.g., combustion-type) reaction of the gas sensing element with the target gas; and (iii) a temperature change of the gas sensing element caused by a different thermal conductivity of the target gas versus ambient gases (this effect is dependent on gas flow over the sensing element). In both chemical-reaction-type gas sensors (i.e., semiconductor gas sensors configured in accordance with reaction type (i)) and in thermal-reaction-type gas sensors (i.e., semiconductor gas sensors configured in accordance with reaction types (ii) and (iii)), the resulting change in electrical resistance across the gas sensing element is measurable by way of passing a current through the gas sensing element, and monitoring the current for changes that are characteristic of reactions with the target gas.
Semiconductor gas sensors have an advantage over other gas sensor types is that they can typically be produced using low-cost photolithographic fabrication processes developed for integrated circuit (IC) fabrication, and are therefore smaller and less expensive than other gas sensor types. In many cases, semiconductor gas sensors utilize bulk Si or SOI wafers as starting materials, and include a thermally isolated membrane formed by removing silicon from the rear of the wafer by plasma or wet etching. The etch stops at the BOX of the SOI or at silicon nitride layer formed at the surface of bulk silicon. The gas sensing element, sensors and a resistive heater are typically formed on the membrane, and control circuitry of the gas sensor is typically fabricated on the adjacent bulk Si or SOI using known semiconductor processing techniques. Typical semiconductor gas sensor designs include close-membrane sensors and membranes suspended by holding arms, with the gas sensing element typically disposed in a center of the membrane to improve thermal isolation.
A problem with conventional chemical-reaction-type semiconductor gas sensors is that they require gas sensing elements that are limited to detecting one or a relatively small number of target gases. That is, the detection mechanism of chemical-reaction-type semiconductor gas sensors requires adsorption of the target gas into the gas sensing element material, and there is no gas sensing element material that is receptive to all gas types. Therefore, chemical-reaction-type semiconductor gas sensors are either limited to one target gas (or a very small number of target gasses), or must include multiple gas sensor units, each unit having a different gas sensing element materials, in order to detect more than one target gas type.
Two chemical-reaction-type semiconductor gas sensors have been proposed in which the gas sensing element is incorporated into a magnetic tunnel junction (MTJ) element. MTJ elements typically include two ferromagnetic electrodes separated by a thin insulating layer and configured such that a resistance across the MTJ element depends on the relative orientations of the easy axes of magnetization (herein “magnetic orientations” or “magnetic directions”) of the two ferromagnetic electrodes. The magnetic orientation of one of the ferroelectric electrodes is typically fixed (e.g., using an adjacent antiferromagnetic layer) and acts as a reference layer of the MTJ element, while the second ferromagnetic electrode forms a “free” layer of the MTJ element whose magnetic orientation can be switched by an external magnetic field between parallel and anti-parallel magnetic orientations relative to the reference layer. When the magnetic orientations of the reference and free layers are parallel, a current conductance through the tunnel dielectric is relatively high (i.e., the MTJ element is in a low resistance state), and when the magnetization vectors are anti-parallel, the current conductance is low (i.e., the MTJ element is in a high resistance state). A first MTJ-type chemical-reaction-type semiconductor gas sensor was disclosed in U.S. Pat. No. 8,826,726 (University of California, 2014), where the free layer of a modified MTJ element was formed with a gas adsorbing magnetic material (cobalt) that was found to cause the magnetic orientation of the free layer to flip from anti-parallel (out-of-plane) to parallel (in-plane) when sufficient amount of a gas to be detected (hydrogen) gas was adsorbed, and where the modified MTJ element could be reset by heating the MTJ element to desorb the gas (i.e., heat is not used during the gas sensing phase). U.S. Pat. No. 9,097,677 (Univ. of Florida, 2015) discloses a second MTJ-based chemical-reaction-type semiconductor gas sensor in which two ferromagnetic structures are separated by a gas-sensitive metallic interlayer (e.g., palladium) such that a magnetic exchange coupling between the two ferromagnetic structures is affected by the amount of hydrogen gas adsorbed into the metallic interlayer. Similar to other chemical-reaction-type semiconductor gas sensors, MTJ-based chemical-reaction-type semiconductor gas sensors are limited in that they can only detect a limited number of gas types. Moreover, quantitative gas measurement using conventional MTJ-based chemical-reaction-type semiconductor gas sensor relies either on determining the time required for a single MTJ element to flip magnetic orientations, or determining magnetization vector angle changes when the temperature is switched from T1 to T2 value, with neither approach providing practical and sufficiently accurate quantitative measurement result data. Further, both U.S. Pat. Nos. 8,826,726 and 9,097,677 fail to specify cooling the MTJ elements to room temperature before measuring resistance in order to determine changes to the MTJ element's magnetic orientation.
Although not limited to a small number of target gasses like chemical-reaction-type semiconductor gas sensors, conventional thermal-reaction-type semiconductor gas sensors are insufficiently accurate in that they require measuring resistance changes at high temperatures. For example, combustion-type semiconductor gas sensors utilize catalyst-type gas sensing elements (e.g., Platinum or Palladium) that are heated to the threshold temperature of a combustible target gas, whereby the target gas exothermically reacts (oxidizes) to further increase the temperature of the catalyst-type gas sensing element to a reaction temperature, which is typically in the range of 100-350° C. Different target gasses have different threshold temperatures, so the catalyst-type gas sensing element is heated to different threshold temperatures at different times to detect different target gasses. Sensing of the reaction temperature increase is performed either by measuring current changes through the gas sensing element materials themselves (e.g., measuring resistance of a platinum catalyst), or by measuring the resistance of a sensor structure formed on the membrane adjacent to the gas sensing element (e.g., a specially formed MOSFET as taught, e.g., in D. Briand et al, IEEE EDL-22(I), pp. 11-13, 2001). Of these options, the special MOSFET approach currently provides the highest sensitivity (i.e., about 2%/° K at room temperature), with most other resistive sensor approaches exhibiting less than 0.5%/° K (IEEE Trans. ED vol. 52, 2005 “Temperature Sensitivity of SOI-CMOS Transistors for use in Uncooled Thermal Sensing”, Eran Socher at al). However, in either case, the resistance measurement must take place during the exothermic reaction, which means that the measuring current must be passed through a measuring element (e.g., the special MOSFET or gas sensing element) that is close to the reaction temperature. Unfortunately, operation of the special MOSFET sensing transistor at high temperatures (above 150° C.) is limited due to leakage currents (even for partially or fully depleted SOI MOS transistors, where leakages are significantly lower than for the bulk devices). Moreover, the thermal sensitivity of the gas sensor's control circuit is strongly decreased at elevated temperatures (e.g., the thermal sensitivity of 0.35 μm transistors was observed as decreasing by approximately 50% at 100° C. as compared to room temperature).
What is needed is a low-cost thermal-reaction-type semiconductor gas sensor that avoids the high-temperature resistance measurement problems associated with conventional thermal-reaction-type semiconductor gas sensors. What is also needed is a low-cost gas sensor capable of quickly and accurately measuring the concentration level of a target (sensed) gas in a gaseous environment.